Mounting scientific evidence shows that animal antibiotics are contributing to a rise in drug-resistant bacterial diseases among humans. But a new study out of Poland has found high levels of these antibiotic-proof pathogens in the natural environment as well. And yet again, animal antibiotics seem to be the culprit. 

Researchers at the University of Warsaw tested soil samples from farmland, vegetable gardens, composted soil and forest soil, finding the greatest diversity  of antibiotic-resistant bacteria in vegetable garden and fruit orchard soils, both of which had been treated with manure. 

This manure came from antibiotic-fed animals, suggesting that these drugs probably contributed to the higher level of antibiotic resistance in the soil.

Trees in both orchards in the study had also been treated with antibiotic sprays in the past to prevent the spread of infection.

Overall, soils not treated with manure, including forest soil and plant compost, carried the lowest levels of drug-resistant microbes. Bacteria resistant to multiple drugs was found only in the vegetable garden soil. 

Antbiotic residues are created naturally in soils, leading to a natural drug-resistance among some microbes; however resistance to human drugs was higher than the naturally occurring rate in soils treated with manure.

This study adds to the body of knowledge that should be used to guide antibiotic policy, says first author Magdelena Popowska.

The findings “should assist in the development of regulations regarding the use of antibiotics in the broader environment e.g. in plant protection products, fish farming and industry,” said Popowska, according to Tips from the Journals of the American Society of Microbiology.

The use of antibiotics “should be restricted to dangerous bacterial infections, and to strict medical supervision,” says Popowska. “This cannot be emphasized strongly enough.”

In the U.S., 80 percent of all antibiotics sold are currently given to food animals for a variety of uses including growth promotion, disease prevention and treatment. 

FDA has issued a draft guidance on the judicious use of antibiotics in animal  production which recommends that drugs important to human medicine not be used for animal growth promotion and be supervised by a veterinarian when used to prevent or treat animal disease.

So far, however, these guidelines have not been finalized. The Food and Drug Administration has said that when it does publish its final recommendations, it hopes that industry will self-regulate and it will not have to impose regulations itself. 

Of the bacterial cultures examined in the study, 42.8 percent were resistant to streptomycin, 34.7 percent were resistant to erythromycin, and 10.2 percent to tetracycline. 

In December 2011 FDA withdrew its plan to ban tetracycline from use in animal feed, announced in 1977. The drug is commonly used to treat MRSA infections in humans. In 2010, more than 12 million pounds of tetracycline were given to animals to promote growth and prevent the spread of disease, while 1.5 million pounds were distributed for animal use.